Procoagulant alpha-thrombin is the central bioregulatory enzyme in hemostasis. Gram quantities of the highly purified human enzyme are being produced in our laboratory for detailed studies on its physical properties, enzymic specificity, and biologic functions. In addition, nonclotting and chemically modified thrombins are being prepared for mapping active-site regions and bracketing the noncovalent protein binding site for fibrinogen recognition. Various collaborative projects include those on cellular receptors for thrombin and its initiation of cellular processes, as well as the pathophysiology of this serine protease. Our recent data on the fibrin(ogen) recognition site of alpha-thrombin explain many of the unique biological activities and perhaps physiological functions of the enzyme.